Diffuse interstitial fibrosis of the lung represents a common clinical, radiologic and pathologic entity. A large number of cases with interstitial fibrosis are designated idiopathic and have been termed cyrptogenic fibrosing alveolitis. Hereditary predisposition, autoimmunity and viral infection have been implicated as etiologic, but their relative importance and mutual inter-relations have not been defined. Hereditary predisposition is suggested by a familial aggregation and several reported series. Whether this predisposition is associated with a particular histocompatibility focus will be determined. Immunopathologic factors have been implicated in certain cases of CFA, which demonstrate circulating non-organ specific antibodies. We have shown that immune complexes consisting of the patient's immunoglobulin and complement are deposited in the lungs of subjects with the cellular forms of CFA. This finding correlates well with the presence of circulating immune complexes. We planned to further study the nature of the circulating immune complexes by a modification of the Raji cell technique. In addition, routine and immunoelectronmicroscopy will be performed to determine the location of these complexes in the alveolar capillary membrane.